Knowledge of the supranuclear connections to motoneurones innervating the extraocular muscles is a prerequisiste to understanding the etiology of strabismus and other disorders affecting oculomotor function. A complete catalogue of the afferent connections of the abducens and accessory abducens nuclei will be compiled by injections of horseradish peroxidase into these structures, resulting in retrograde transport and labelling of all brain stem neurones projecting to these nuclei. The morphology and distribution of afferent synaptic endings derived from the brain stem supranuclear structures and terminating upon abducens neurones will be determined by injections of tritiated amino acids into the various afferent nuclei followed by electron microscope autoradiographic examination of ultrathin sections through the abducens nucleus. Histochemical studies of human overacting inferior oblique muscles wll be undertaken to determine which muscle fiber types show changes as a result of overaction and what changes have occurred in the metabolic functions of specific organelles within the affected fibers. The distribution of each of the muscle fiber types will be reconstructed, with partiular emphasis upon the organization of muscle fibers as it relates to the anterior and posterior segments of the normal inferior oblique muscle.